The conventional internal combustion engine has a reciprocating piston in a cylinder liner. The piston motion has two dead positions at the top and the bottom (TDC and BDC). The oil film between the piston skirt and the cylinder liner breaks down when the piston reverses its direction of motion at both TDC and BDC. Oil film must be redeveloped and the coefficient of friction in sliding under oil boundary lubrication is high and around 0.1. The frictional power loss of all the pistons in the cylinder liners has accounted for almost 40% to 50% of all the engine's frictional loss. If an engine has a mechanical efficiency of 83%, the frictional loss at the piston will be 6.8% to 8.5% of the engine's available horsepower.
A natural way to reduce the sliding frictional loss due to redeveloping an oil film between the piston skirt and the cylinder liner is to convert the sliding motion of the piston into a rolling motion with less friction. The coefficient of friction for pure rotation in oil is only 0.001 to 0.1 for a ball or roller. Ball bearings should be installed on the piston skirt. Such an idea leads to this invention of an anti-thrust ball bearing piston.
1. Field of the Invention
This invention relates to the field of anti-friction pistons. The reduction of friction has been achieved through a piston with rollers in the skirt, a two piece piston with cylindrical barrel rollers or a spherical ball bearing, and others.
2. Description of the Prior Art
Most of the prior patents utilized the rollers in the piston skirt or in the cylindrical barrel to withstand the piston thrust load under the peak firing pressure of the engine. Some patents use a single big spherical ball bearing. They all convert the sliding motion of a piston into the rotating motion.
U.S. Pat. No. 4,704,949, Piston, to John D. Foster in 1987 uses two sets of three roller bearings mounted on a light weight piston skirt. It has the same advantages of the piston of U.S. Pat. No. 3,398,653 and reduce the required number of roller bearings to three. Each roller has an axle, a roller case and five small rollers within the case and around the axle. It seems that the case hardening of the piston skirt and the cylinder liner may have been considered. But the load carrying capacity of three axles will limit the maximum allowable thrust load of the piston and the engine horsepower.
U.S. Pat. No. 4,704,949, Peristrophic Internal Combustion Engine Assembly and Multi-Part Pistons, to Theodore Koutsoupidis in 1989 uses a two-part piston. The lower bearing piston may have a circumferential ring, cylindrical barrel rollers, or a spherical ball bearing arrangement. Either the rollers or spherical ball bearing will withstand the piston thrust load. Less piston friction due to bearing rotation will be achieved. But the piston becomes multiple parts. Also a single ball bearing has its load carrying capacity and will limit the engine's power rating.
Whatever the precise merits, features and advantages of the above cited references, these pistons are relatively complex and only cover limited applications due to the load carrying capacity of the rollers or ball. When the number of rollers or balls is reduced, the contact stress will be increased if the diameter of the roller and ball remains the same. The contact stress must be less than or equal to the contact strength of the cylinder liner or piston skirt in contact. Otherwise, the engine life will be sacrificed.